Worst must be APB (MMOish fps), got bored of it on the first day and shortly after it went bust and servers went down.
Best probably Fallout (1, 2, 3 and New Vegas) or like DeadEyeDuk says CS.
So many hours spent on CS, wether it was on public servers or league matches, actually played CS:S again on saturday for the first time in a year or so and it's still great fun (although really annoying to see how bad I am at it now).
Will probably end up playing CS:GO when it comes out.
Another nice thing about CS is that this is where I first met my GF.